This invention relates generally to local power monitoring assemblies of the type disclosed in prior co-pending application, Ser. No. 888,881, filed Mar. 21, 1978, now U.S. Pat. No. 4,298,430 and owned in common with the present application. More particularly the present invention is an improvement residing in the differential temperature sensing arrangement associated with such power monitoring assemblies.
Generally, two signal producing junctions of a thermocouple device have been utilized in each of the measurement zones monitored by gamma ray sensors to which the aforementioned prior application relates. The thermocouple junctions have been located within a central bore of the elongated heat conductive body of the sensor at axially spaced locations within adjacent hot and cold regions of each measurement zone to produce a differential temperature signal from which heat flow rate may be determined reflecting local power generation in a nuclear reactor. In an axial heat flow arrangement, a hot junction is located centrally within the hot region of the zone or midway between the adjacent cold regions on the assumption that a symmetrical temperature gradient is established within the measurement zone with a peak temperature occurring at the location of the hot junction. However, conditions may develop in such heat flow rate monitoring assemblies which cause asymmetrical axial heat flow in the hot region producing an asymmetrical temperature distribution and a shift in the peak temperature point from the axial center location of the hot junction as well as a temperature differential between the cold regions. The differential temperature signal produced by the two thermocouple junctions will thereupon be affected by a signal error which makes heat flow rate determination inaccurate.
Furthermore, the foregoing two junction thermocouple arrangements depend on substantial gamma radiation generated heating for the production of a signal of measurable strength particularly where there is a noise level problem. In connection with radial heat flow types of sensors utilized in boiling water reactor installations, there may be a signal strength problem. Heretofore, gas chambers and multi-component units have been utilized to provide signal gain in such a situation.
Another problem associated with such two junction thermocouple arrangements resides in signal response to changes in heat flow rate. This is of particular concern in boiling water reactor installations where the signal level must be sufficiently high to trigger safety circuits. To increase signal response speed, the mass of the heat conductive body in the measurement zone may be reduced. However, such radiation in mass results in a decrease in signal strength toward noise level and is an unsatisfactory compromise.
It is therefore an important object of the present invention to provide a differential temperature sensing device for heat flow rate monitoring assemblies which will cope with the aforementioned problems of signal error due to asymmetrical heat flow, low signal strength, and signal response to changes in heat flow rate.